Anodising of aluminium and its alloys



United States Patent 3,472,744 ANODISING OF ALUMINIUM AND ITS ALLOYS James M. Kape, West Molesey, Surrey, England, assignor to Acorn Anodising Company Limited, London, England, a company of Great Britain No Drawing. Filed Dec. 5, 1966, Ser. No. 598,933 Claims priority, application Great Britain, Dec. 9, 1965,

52,347/65 Int. Cl. C23b 9/02 US. Cl. 204-58 ABSTRACT OF THE DISCLOSURE This invention relates to a method of anodising aluminium and its alloys in order to produce hard and coloured coatings using an electrolyte incorporating a first acid selected from dibasic and polybasic organic acids other than oxalic acid with the addition of either glyoxylic or oxalic acid.

This invention relates to a method of anodising aluminium and its alloys.

It is well known that if aluminium is made the anode in aqueous solutions of malonic acid and an electric current is passed under an applied potential an anodic oxide film is forme-d on the aluminium. Extremely hard anodic coatings may be produced which range in colour from deep yellow to sepia brown and which have been particularly described in an article by J. M. Kape on pages 181-191 of Metallurgia of November 1959.

Using the malonic acid electrolyte it is found that exeremely high applied voltages (over 120 volts) are required to cause even small current densities (10-15 amperes per square foot) to pass at ambient temperatures in order to promote the formation of thick coloured anodic coatings.

Further, the use of the methods described in the literature mentioned above according to which the temperature of the electrolyte was raised to 40-60 C. (preferably 50-55 C.) in order to cause the production of current densities of -24 amperes per square foot at applied voltages of less than 100 volts, is disadvantageous in that the abrasion resistance of the anodic coating is substantially reduced and the density of colouration of the coating per unit film thickness is considerably reduced. Moreover, at these temperatures a considerable proportion of the malonic acid is decarboxylated according to the equation:

The chance of this reaction occurring is considerably minimised by reducing the temperature but at a temperature of C. it is found that somewhat rough uneven coatings are produced on certain aluminium alloys-notably extrusions. In particular, on an alloy known under the nomenclature of British Standards specifications Nos. 1470 and 1490 as HE9 which is commonly used for building purposes, coatings are obtained which show up scratches and die lines as whitish streaks and thus are valueless for decorative applications.

If aqueous solutions of other organic dibasic or polybasic acids are used as the electrolyte then in general only thin anodic coatings are produced the thickness of such coatings being roughly proportional to the applied voltage. In some cases instead of forming an anodic coating aqueous solutions of certain dibasic or polybasic organic acids produce pitting of aluminium anodes under an applied potential.

It is also well known that if an aqueous solution of oxalic acid is used as the electrolyte then an anodic coating can be produced under favourable voltage and current density conditions on aluminium alloys which are free 3 Claims 3,472,744 Patented Oct. 14, 1969 ice from copper, this coating being very hard and coloured pale yellow or brown the resultant anodised material having particular application architecturally.

It is an object of the present invention to provide an improved method of anodising aluminium and its alloys to produce particularly hard anodic coatings within acceptable limits of applied voltage and at adequate current densities with a wide range of colours.

According to the present invention there is provided a method of anodising aluminium and its alloys in which the anodising operation is carried out in an electrolyte comprising a first acid selected from dibasic and polybasic organic acids other than oxalic acid with the addition of a second acid different from said first acid and selected from oxalic acid and glyoxylic acid,

Preferably the first acid is selected from the group consisting of malonic acid, tartaric acid, citric acid, diglycollic acid, malic acid, succinic acid, glutaric acid, itaconic acid, maleic acid, pyromellitic acid, trimellitic acid, citraconic acid, thiomalic acid, thiodiglycollic acid, acetylene dicarboxylic acid, aconitic acid, glyoxylic acid and phthallic acid. In this specification glyoxylic acid is to be considered as a dibasic acid. The first acid may be a solution having a concentration ranging from 1% to saturation and the amount of the second acid incorporated in the electrolyte may vary between 0.1% and 10% by weight of the first acid.

The following examples illustrate the invention. Experiments were carried out using HE9 (British Standards specifications 1470 and 1490) test pieces, a current density of about 30 amperes per square foot for 1 hour andan electrolyte temperature of 20 C. Coating thicknesses of 30-40 microns were obtained and the results are tabulated below, M representing malonic acid and Ox representing oxalic acid.

Voltage Electrolyte range Type of Coating 10% M 125-135 Dark brown, non-uniform. 10% M+0.2% 0x 130-140 Dark brown, more uniform. 10% M+0.4% Ox.-- -130 Dark bronze, fairly smooth. 10% M+0.6% Ox 110-135 Dark bronze, smooth. 10% M+1.0% Ox.-. 100-115 Bronze, smooth, uniform. 10% M+3.0% Ox 85-100 Light bronze, smooth, uniform. 10% M+5.0% Ox...-. 70-00 Very light bronze, smooth uniform.

When the additions of oxalic acid reached or exceeded 3.0% by weight, the coating produced resembled that produced with an electrolyte containing oxalic acid alone.

Similar experiments were carried out but modifying the malonic acid by an addition of glyoxylic acid (represented by G). The results are shown in the table below (HE9 material, 20 C., approximately 30 amperes per square foot for 1 hour, film thicknesses 30-40 microns).

Voltage Electrolyte range Type of Coating 10% M 130-145 Dark brown, non-uniform. 10%M+0.5% G 130-135 Dark brown more uniform. 10% M +10% G 125-135 Dark bronze, smoother. 10% M +2.0% G 125-140 Dark bronze smooth. 10% M 3.0% G -145 D0.

Typical specific abrasion resistance of a 25-35 Temperature micron thick coating gms./ Electrolyte micron film Electrolyte 0.) thickness.

10% oxalic acid 20 8. 7 10% malonic acid 20 6. 6 10% malonic acid+0.2% oxalic acid- 20 5. 25 10% malonic aeid+0.6% oxalic acid 20 5.0 10% malonic acid+1.0 glyoxylic acid. 20 8. 17.5% by vol. H SO4+1% oxalic acid 0 4.0 7.0% by vol. sulphuric acid 20 2. 0 10% sulphosalicylic acid-{05% by wt.

H 804 25 4. 0 sulphophthallic acid+0.5% by wt.

It can be seen that the malonic acid-oxalic acid electrolyte and the malonic acid-glyoxylic acid electrolyte each afford much more abrasion-resistant coatings than other processes used for architectural anodising and hard anodising.

Similar experiments were carried out using an electrolyte incorporating various other polybasic organic acids and the results are tabulated below with the inclusion of comparison figures for a sulphuric acid electrolyte:

1. A method of anodising aluminium and its alloys in which the anodising operation is carried out in an electrolyte consisting essentially of a first acid in a solution having a concentration ranging from 1% to saturation and selected from the group consisting of malonic acid, citric acid, diglycollic acid, malic acid, succinic acid, glutaric acid, itaconic acid, maleic acid, pyromellitic acid, trimellitic acid, citraconic acid, thiomalic acid, thiodiglycollic acid, acetylene dicarboxylic acid, aconitic acid, glyoxylic acid and phthallic acid with the addition of a second acid different from said first acid and selected from oxalic acid and glyoxylic acid in an amount which varies between 0.1% and 10% by weight of the first acid.

2. A method of anodising aluminium and its alloys in which the anodising operation is carried out in an electrolyte consisting essentially of malonic acid having a concentration of 10% with the addition of oxalic acid in an amount varying between 0.6% and 5% by weight of the malonic acid.

3. A method of anodising aluminium and its alloys in which the anodising operation is carried out in an electrolyte consisting essentially of malonic acid having a concentration of 10% with the addition of glyoxylic acid in anamount ranging between 0.5% and 3% by Weight of the malonic acid.

Thickness Specific 1 Voltage of coatin abrasion Electrolyte range Colour of coating (microns resistance 8% sulphuric acid 19-20 Silver 2-35 2.0 6% tartaric+1.5% oxalic 100-130 Brown-grey 20 7. 5 5% tartaric-W3 oxalic- 110-155 Medium grey- 20 6. 5% citric+1.5% oxalic -104 Greenish-yellow 25 6.9 5% citric+0.3% oxalic -180 Grey-brown. 15 8. 3 5% itaconic+1.5% oxalic" -150 Grey 25 6.9 5% maleic+1.5% oxalic. 92-130 Grey-bronze. 27 8.1 5% malic+1.5% oxalic. 180-200 Dark grey. 25 7. 45 5% diglycollic+1.5% oxalic 100-130 Yellow grey 25 7. 25 5% diglycollic+1.2% oxa1ic 100-140 Yellow-brown- 30 10. 5 5% thiomalic+1.5% oxalic. 88-162 Grey-bronze 25. 0 7. 6 1% plthallic acld+2% oxalic 74-06 Medium grey to gold.. 25. 0 9. 0

aci 2% trimellitic acid+0.75% 50-140 do 15. 0 9. 0

oxalic aci 1% citraconic acid+2% 78-100 do 30. 0 7. 7

oxalic ac 2% pyromellitic aeid+0.3% -175 do 12. 5 9. 4

xalic acid.

1 After sealing5 minutes in Nickel Acetate 8 grams/litre, 80 0. plus 60 minutes hot water 95 C.

References Cited UNITED STATES PATENTS 672,913 4/1931 Pollak 204--58 1,965,682 7/1934 Worm 204-58 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 

